Socket-outlet

ABSTRACT

A socket-outlet, in which, only in response to preselected loads, voltage from the supply-side is connected through by a switching device to the load side. A voltage-supplied measuring device has a D.C. testing circuit and an A.C. testing circuit. Both testing circuits attempt to drive measuring currents across a connected load. The A.C. testing circuit operates at higher frequencies than the power-line frequency. An evaluation circuit is designed, on the basis of preselected criteria and on the basis of measuring values, in logical AND-connection, to determine whether switching contacts of the switching device should be closed.

FIELD OF THE INVENTION

The present invention relates to a socket-outlet, in which voltage isconnected by a switching circuit, from a supply side to a load side onlyin response to a designated load. Thus, voltage will be provided at thesocket-outlets only when an electrical load is connected. German PatentApplication No. U 9 312 528 describes a bus system which directs aswitching command to a socket-outlet. In this way, the socket-outlet isusually off-circuit (i.e., without power), and thus that in-circuit(i.e., with power) parts cannot be reached by small children, ininadvertent contact, or by adults, during wallpapering or resulting froman unscrewed cover. A voltage-supplied measuring device includes a D.C.testing circuit which is designed to drive a current across aconnectable load for measuring purposes. The D.C. testing circuit isdesigned to measure the impedance of a connected load using a testvoltage that is low with respect to a dangerous voltage. In addition,the device includes an A.C. testing circuit which is designed to measurethe impedance of a connected load using a frequency which is at leasthigher that the line voltage, and at a testing voltage that is lowerthan a dangerous voltage.

PCT Application No. WO 89/11747 describes a safety device which isswitched on between a power supply and a load to be protected. In thisdevice, testing voltages of 5 volts and 9 volts are used for animpedance measurement in order to protect living creatures.

SUMMARY OF THE INVENTION

The present invention is directed to an off-circuit socket-outlet whichis independent from external supplied switching commands.

In accordance with the present invention, a voltage supplied measuringdevice includes a D.C. testing circuit, which is designed to permanentlydrive a current across a connectible load, the D.C. testing circuitbeing designed to measure the impedance of a connected load using a testvoltage that is low with respect to a possible, dangerous voltage. Analternating current of the measuring device is also provided, thealternating current being at a high frequency, or higher in comparisonto the line voltage, and at a low voltage with respect to a possibledangerous voltage. An evaluation circuit, including microprocessors or aCPU, is designed to decide, based on preselected criteria and measuringvalues supplied by the measuring device, whether the switching contactof the switching device should be connected. A socket-outlet of thistype offers should be connected. The evaluation circuit is designed soas not to provide a connection in response to an Impedance 1 of morethan 500 Ohms, measured by the D.C. testing circuit, logically ANDed tothe relation:

1.3≦Impedance 1/Impedance 2≦15

Impedance 2 being determined by the A.C. testing circuit. The prohibitedrange, in which a connection is not permitted, corresponds to thecriteria defined for a human being. The criteria can be adjusted forother living beings. A socket outlet of this type offers current circuitbreaker cannot respond because the entire current is flowing back andforth between the monitored conductors. A situation of this type couldarise, for example, when a child, insulated from ground, makes contactwith the socket-outlet, in the outer conductor and the return conductor,by inserting nails. Heretofore, it has been attempted to avoidendangerments of this type using shutters with mechanical means keepingthe socket-outlet closed as long as contact means are not introducedsimultaneously into the connecting openings, such as occurs with a plugand its plug contact pins. However, the socket-outlet proposed here issafer than a shutter of that type, which in unfavorable circumstances,in response to certain actions, can be outsmarted. For entry purposes,suitable criteria is described in literature, for example, the book byGottfried Bieglmeier, Wirkungen des elektrischen Stromes auf Menschenund Nutztiere (The Effects of Electrical Current on Humans and WorkingAnimals).

In practice, it is advantageous if the D.C. testing circuit of themeasuring device operates using a voltage in the order of magnitude of 5volts. For the A.C. testing circuit of the measuring circuit, it isbeneficial to select a frequency in the order of magnitude of 10 kHz anda measuring voltage of 5 volts, to ascertain the impedance after apredetermined time period.

It is advantageous to design the evaluation circuit so as not to connectthrough in response to an Impedance 1 of more than 500 Ohms, measured bythe D.C. testing circuit, in logical AND-connection to the relation,

1.3≦Impedance 1/Impedance 2≦15,

Impedance 2 being determined by the A.C. testing circuit. The prohibitedrange, in which through-connecting is not permitted, corresponds to thecriteria defined for a human being. The criteria can be adjusted toother living beings.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE illustrates a socket-outlet safety device in accordance withan exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the FIGURE, a socket-outlet 1 is illustrated, to which a load 2 isconnected. On the supply side, provision can be made for conventionalscrew connections or also for screwless connections, and on load side 4,plug-in socket-outlets 4 for plug pins. In this context, socket-outletmeans all other kinds of plug connections that ensure an analogoussafety for the elements to be connected.

A measuring device 5 is supplied with voltage by a power supply unit 6.Measuring device 5 has a D.C. testing circuit 7 and an A.C. testingcircuit 8. D.C. testing circuit 7 and A.C. testing circuit 8 cooperatewith an evaluation circuit 9. Evaluation circuit 9 can be implemented onthe basis of a microprocessor or, generally, as a data processing devicehaving a CPU. When evaluation circuit 9, on the basis of the measuringvalues of D.C. testing circuit 7 and A.C. testing circuit 8 incomparison to the preselected criteria, observes the safety requirementsas being fulfilled, an actuator 10 is acted upon, which conductivelycontrols, i.e., through-connects, a switching device 11. Socket-outlet 1on supply side 3 has connections for an external conductor L, and for aneural conductor N, and for a protective conductor PE. In the drawing,at load side 4, a load 2 is connected, whose equivalent circuit diagramis depicted by a resistor 12. Power supply unit 6 delivers the on-boardline voltage for actuator 10, for evaluation circuit 9, D.C. testingcircuit 7, and A.C. testing circuit 8.

D.C. testing circuit 7 permanently seeks to drive a current across aload 2 or its equivalent resistance 12. In response to a connected load,a D.C. current can flow and be measured.

Before the switching contacts of switching device 11 are closed, it isascertained by measuring device 5 whether an electrical load has thecorresponding criteria. For example, if the impedance of resistor 12 ismeasured by D.C. testing circuit 7 in response to a measuring voltage of5 volts and if, for example, a measuring value of about 100 ms isevaluated and, subsequently, impedance two of resistor 12 is measured byA.C. testing circuit 8 at 10 KHz at a measuring voltage of 5 volts andis evaluated at about 100 ms, then no connect-through will occur, if thefollowing conditions are met:

Impedance one, measured by D.C. testing circuit 7, is greater than 500Ohms, i.e., impedance 1>500 Ω; in a logical AND-connection to1.3≦Impedance 1/Impedance 2≦15. The range where both conditions aresatisfied defines the prohibited range, which corresponds to thecriteria as they are given for humans. Outside the prohibited range, thethrough-connection takes place. The criteria can be selected in such away that various body sizes, for example, of children and adults, can betaken into account, that dry and wet skin, depending on the area ofapplication, can be taken into account. Corresponding minimal andmaximal values for the impedance can be entered. The measuring pointsusing D.C. voltage and A.C. voltage can be selected by the D.C. testingcircuit and the A.C. testing circuit in such a way that thecurrent-compatibility of an organism to be protected is taken intoaccount in the case of D.C. current and at 10 KHz. To protect persons,testing currents should be employed well within the release limit.

We claim:
 1. A safety system for a socket-outlet, the socket-outlethaving a supply side and a load side, the supply side coupled to a firstvoltage source, the load side connectable to a load, comprising:aswitching device for coupling the supply side to the load side; and ameasuring device coupled to the load side and including a D.C. testcircuit, the D.C. test circuit for driving a current across the load andfor measuring a first impedance of the load using a low test voltage,the measuring device further including an A.C. test circuit formeasuring a second impedance of the load using a frequency that is highcompared to a line voltage, the measuring device further for controllingthe switching device to not couple the supply side to the load if i) thefirst impedance is greater than 500 ohms, and ii) a relation: 1.3≦firstimpedance/second impedance≦15 is satisfied.
 2. The safety systemaccording to claim 1, wherein the low test voltage is in an order of 5volts.
 3. The safety system according to claim 1, wherein the frequencyhas a magnitude in an order of 10 kHz, the A.C. test circuit has ameasuring voltage in an order of 5 volts, the A.C. test circuit fordetermining the second impedance within a predetermined time period.